J Nondestruct Eval (2011) 30:81–90 DOI 10.1007/s10921-011-0093-9 A Dielectric Sensing Approach for Controlling Matrix Composition During Oxide-Oxide Ceramic Composite Processing D.D. Hass · H.N.G. Wadley Published online: 24 February 2011 © Springer Science+Business Media, LLC 2011 Abstract Continuous fiber reinforced ceramic matrix com- posites (CMC’s) made from aluminum oxide fibers and ma- trices are usually fabricated using a tape casting process. In this process, ceramic slurry consisting of the oxide powder, a polymeric binder and a solvent is infiltrated into a wo- ven ceramic fiber mat. After evaporation of some of the sol- vent, the resulting flexible tapes can be stacked and sintered to create a composite component. Because the fraction of ceramic powder in the slurry can vary during processing, in-situ compositional sensors are required for on-line feed- back control to limit property variations in the composite material. Since the dielectric properties of the slurry com- ponents are distinctly different, the effective permittivity of the slurry depends upon its composition. Here, a non-contact capacitance probe has been used to explore the possibility of capacitance sensing for compositional control. Results indi- cate that the removal of solvent during a precision drying step may be monitored by this approach. The feasibility of monitoring changes in the slurry’s composition during infil- tration of the fiber mat is also discussed. Keywords Ceramic-matrix composites (CMCs) · Slip casting · Process sensing · Non-destructive testing 1 Introduction Many ceramic structures and devices are fabricated from ce- ramic powders using slurry-based methods such as tape cast- ing [1]. In this approach, the ceramic powder is dispersed D.D. Hass · H.N.G. Wadley ( ) Department of Materials Science and Engineering, School of Engineering and Applied Science, University of Virginia, Charlottesville, VA 22903, USA e-mail: haydn@virginia.edu in a liquid solvent/binder mixture, formed into a thin tape, dried, stacked and finally sintered at high temperature. Ex- amples of materials made this way include piezoelectric ce- ramics [2], multilayer capacitors [3], solid oxide fuel cells [4], current collectors in batteries [5], and ceramic matrix composites (CMC’s) [6]. The study described here is moti- vated by a continuing interest in the development of ceram- ics for high temperature applications in the hot sections of gas turbine engines [7]. While monolithic ceramic materials based upon alumina, silicon carbide, or silicon nitride have a lower density, higher stiffness, lower creep rate, and (in the case of oxide-based ceramics) greatly reduced susceptibility to oxidation com- pared with conventional superalloys [8], their low tough- ness has restricted widespread application [9]. Ceramic ma- trix composites (CMC), in which strong continuous ceramic fibers are embedded in a ceramic matrix can result in a more damage tolerant mechanical behavior due to crack deflec- tion, fiber pull-out, crack bridging and de-bonding mech- anisms [10, 11]. The emergence of oxide fibers such the alumina based Nextel 440, 610 and alumina with mullite (2SiO 2 3Al 2 O 3 ) Nextel 720 fibers and alumina based matri- ces has led to the development of oxide-oxide CMC’s [12] and interest in their use for a growing number of high tem- perature applications [13–15]. However, reliable processing methods are needed if the potential of these materials is to be fully realized. The tape casting approach is widely used for making oxide-oxide composites since equipment costs are low and production rates are potentially high [16]. The process, shown schematically in Fig. 1, involves the infiltration of a woven ceramic fiber mat by a slurry containing ceramic powder, a polymeric resin and organic solvent of a con- trolled composition and viscosity. The infiltrated mat is then precision dried so that a controlled fraction of the solvent is